The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera to be built in a cellular phone, a portable information terminal, or the like, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones”, i.e., multifunctional cellular phones which can run various application software as well as a voice call function, have been more widely used. When application software is run on smartphones, it is possible to perform functions such as those of digital still cameras and car navigation systems on the smartphones. In order to perform those various functions, most models of smartphones include cameras.
Generally speaking, product groups of such smartphones are often composed according to specifications for beginners to advanced users. Among them, an imaging lens to be mounted in a product designed for the advanced users is required to have a high-resolution lens configuration so as to be also applicable to a high pixel count imaging element of these years, as well as a small size.
As a method of attaining the high-resolution imaging lens, there has been a method of increasing the number of lenses that compose the imaging lens. However, the increase of the number of lenses easily causes an increase in the size of the imaging lens. Accordingly, in development of the imaging lens, it has been necessary to achieve high resolution while shortening a total track length (TTL) by restraining the increase of the number of lenses or by other method.
In these days, with significant advancement in achieving the higher pixel count of an imaging element and image processing technology, an imaging lens has been developed so as to attain higher resolution rather than a shorter total track length of the imaging lens. For example, there is a camera unit, which is configured separately from a smartphone, and is composed of a high-resolution imaging lens, an imaging element, etc. By attaching the camera unit onto a smartphone, it is achievable to obtain images having equivalent quality to those of high-end model digital still cameras. However, because of the presence of the camera unit, portability of the smartphone is ruined. Therefore, a smartphone with a built-in camera is superior to such smartphone with an attached camera unit in its convenience and portability. Accordingly, there remains a demand for a small-sized high resolution imaging lens.
In case of a lens configuration composed of seven lenses, due to the large number of lenses of the imaging lens, it has high flexibility in design. In addition, it is achievable to attain satisfactory correction of aberrations, which are necessary for high-resolution imaging lenses, and downsizing of the imaging lens in a balanced manner. For example, as the conventional imaging lens having the seven-lens configuration, an imaging lens described in Patent Reference has been known.
Patent Reference: Japanese Patent Application Publication No. 2012-155223
The conventional imaging lens described in Patent Reference includes a first lens that has a shape of a biconvex shape, a second lens that has a shape of a biconcave shape joined to the first lens, a third lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to the object side, a fourth lens that is positive and has a shape of a meniscus lens directing a concave surface thereof to the object side, a fifth lens that is negative and directs a convex surface thereof to the object side, a sixth lens that has a biconvex shape, and a seventh lens that has a biconcave shape, arranged in the order from the object side. According to the conventional imaging lens of Patent Reference, the first through the fourth lenses compose a first lens group, and the fifth through the seventh lenses compose the second lens group. With the configuration, by restraining the ratio of a focal length of the first lens group to that of the second lens group within a certain range, it is achievable to downsize of the imaging lens and satisfactorily correct aberrations.
In case of the conventional imaging lens of Patent Reference, although the size of the imaging lens is small, correction of the image plane is insufficient and the distortion is especially large. Therefore, there is a limit by itself to achieve high performance imaging lens. With the lens configuration of the imaging lens of Patent Reference, it is difficult to achieve satisfactory aberration correction while downsizing of the imaging lens.
Here, such a problem is not specific to the imaging lens to be mounted in cellular phones and smartphones. Rather, it is a common problem for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain both downsizing thereof and satisfactory aberration correction.
Further objects and advantages of the present invention will be apparent from the following description of the invention.